Allow omitting one or both boundaries in an array slice specifier.

Omitted boundaries represent the upper or lower limit of the corresponding
array subscript.  This allows simpler specification of many common
use-cases.

(Revised version of commit 9246af6799)

YUriy Zhuravlev

src/include/nodes/primnodes.h

@@ -341,6 +341,9 @@ typedef struct WindowFunc
  * reflowerindexpr must be the same length as refupperindexpr when it
  * is not NIL.
  *
+ * In the slice case, individual expressions in the subscript lists can be
+ * NULL, meaning "substitute the array's current lower or upper bound".
+ *
  * Note: the result datatype is the element type when fetching a single
  * element; but it is the array type when doing subarray fetch or either
  * type of store.
@@ -360,7 +363,7 @@ typedef struct ArrayRef
 	List	   *refupperindexpr;/* expressions that evaluate to upper array
 								 * indexes */
 	List	   *reflowerindexpr;/* expressions that evaluate to lower array
-								 * indexes */
+								 * indexes, or NIL for single array element */
 	Expr	   *refexpr;		/* the expression that evaluates to an array
 								 * value */
 	Expr	   *refassgnexpr;	/* expression for the source value, or NULL if